1. Field of the Invention
The present invention relates to a technology for encryption of given data and, more specifically, to a technology for reducing cost of the encryption devices as well as for preventing increase in the ease of decryption by a third party and slowdown of encryption processing speed.
2. Description of the Related Art
In Patent Document 1, to be cited later, a memory device, for preventing stored data from being read out for misuse, is disclosed. The memory device adopts a configuration that data to be stored is encrypted and is stored in a high-capacity flash memory.
In the memory device, key data, which is a common key used as an encryption key in data encryption, is stored in a low-capacity memory for control unit, provided in a control unit of the flash memory. In this memory also, data encryption/decryption is performed in the decryption circuit configured in the above control unit. In such performance, the above key data is read from the memory for control unit and is used.
Patent Document 1:
Japanese laid-open disclosure public patent bulletin No. 2001-22646
In order to decrypt the encrypted data stored in the flash memory afterward, the same key data as the one used for the encryption is required, and for that reason, key data stored in the memory for control unit must be preserved even when power supply to the memory device is interrupted. However, it is desirable that key data can be changed to reduce the danger that the encrypted data can be decrypted by a third party as much as possible.
For high-speed performance of encryption and decryption processing, reading speed of the key data from memory for control unit is required to be sufficiently fast so that this speed does not bottleneck the entire processing speed.
In the past, in implementation of the memory device disclosed in the above Patent Document 1, when providing the control unit of the flash memory as a single chip integrated circuit element, in order to meet all of the above requirements, FeRAM (Ferroelectric Random Access Memory) is adopted and formed as memory for control unit to store the key data. However, adoption of the FeRAM raises manufacturing cost of the integrated circuit elements, and consequently increases the cost of the whole memory device.
Instead of storing the key data in the FeRAM formed on integrated circuit elements, a method, in which the key data is stored in a storage area of a part of the above flash memory or storage area of the other prepared flash memory, is also possible. However, data reading speed from the flash memory is considerably slower compared with the reading speed from FeRAM; therefore there is a concern about slowdown of the encryption processing and decryption processing.
In this method, there is also a danger of direct abstraction of the key data from the flash memory by a malicious third party, and the encrypted data can be decrypted using the key data.